1. Field of the Invention
The invention relates to a process for the preparation of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethanol starting out from a mixture containing 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethanol (p,p'-dicofol) together with at least one compound selected from a first group formed by 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2,2-trichloroethanol (o,p'-dicofol); 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2,2-trichloroethane (o,p'-DDT); and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2-dichloroethylene (o,p'-DDE).
The starting mixture, apart from the p,p'-dicofol and one or more of the compounds o,p'-dicofol, o,p'-DDT and o,p'-DDE, may obviously contain other compounds. Particularly, said starting mixture may be 1,1-bis(chlorophenyl)-2,2,2-trichloroethanol, also known as technical dicofol.
Dicofol is an acaricide widely used for agricultural purposes in cotton and fruit crops, citrus fruits in particular.
The biologically active product is 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethanol which is to be found in technical dicofol in concentrations of around 70-75 wt %.
2. Prior Art Reference
The preparation of the product (dicofol) is described in U.S. Pat. No. 2,812,280 (Nov. 1957), 2,812,362 (Nov. 1957) and GB 831,421 (March 1960).
The process initially used consisted of photochemical chlorination (Scheme A) of technical DDT, a mixture of various products, but the main components of which are 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (p,p'-DDT; approx. 75 wt %) and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2,2-trichloroethane (o,p'-DDT; approx. 20 wt %) by introduction of chlorine gas in DDT under fusion ##STR1## which led to the corresponding chlorine derivatives 1,1-bis(4-chlorophenyl)-1,2,2,2-tetrachloroethane (p,p'-ClDDT) and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-1,2,2,2-tetrachloroethane (o,p'-ClDDT).
The chlorination of the o,p'-DDT isomer is incomplete for steric hindrance, leaving a residue of this substance representing a content of about 2 to 7 wt % in the chlorinated end product.
The mixture resulting from the chlorination was subsequently subjected to hydrolysis in an acid medium (Scheme B), consisting of heating to between 120.degree. and 150.degree. C. with an aqueous solution of sulphuric acid and an aryl sulphonic acid. ##STR2##
The dicofol was extracted by dissolution in a solvent and decantation, the organic phase was washed with water and the dicofol was recovered from the solvent by distillation.
The o,p'-DDT was not affected by this hydrolysis and therefore the final content thereof in the technical dicofol was about 2 to 6%.
With a view to reducing this content, the manufacturing process was modified by changing the chlorination by substitution of the DDT by dehydrochlorination first (Scheme C), with a strong alkali, to the corresponding ethylene derivatives, 1,1-bis(4-chlorophenyl)-2,2-dichloroethylene (p,p'-DDE) and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2-dichloroethylene (o,p'-DDE), ##STR3## followed by addition chlorination (Scheme D), also photochemical, to reach the same chlorinated derivative as in the previous case ##STR4## which was subjected to the same acid hydrolysis process.
Nevertheless, the dicofol prepared by any of these processes contains a number of impurities, some of them DDT-related, called DDT.R, among which the following are of known nature. ##STR5##
Other known impurities contained in dicofol and not consisered as DDT.R are 4,4'-dichlorobenzophenone (p,p'-DCBF), 2,4-dichlorobenzophenone (o,p'-DCBF), 4,4'-dichlorobenzyl (p,p'-DCBZ) and 2,4'-dichlorobenzyl (o,p'-DCBZ). ##STR6##
The presence of these impurities implies ecological drawbacks, particularly those related with DDT, generically known as DDT.R, the content of which has been subjected to restrictive specifications to a maximum of 0.1 wt % requiring purification thereof.
U.S. Pat. No. 4,705,902 (Nov. 1987) and Spanish patents 8603248 (16.11.88) and 8802818 (28.07.89) describe purification processes for technical dicofol by liquid-liquid extraction allowing the DDT.R content to be reduced below 0.1 wt %.
Nevertheless, the active principal (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethanol (p,p'-dicofol)) content is still low, between 75-80%, meaning an unnecessary contribution to the environment of additional products in the application thereof.